1. Field of the Invention
The present invention is directed to a method and an apparatus for magnetic resonance imaging (MRI), and in particular to such a method and apparatus making use of the known PROPELLER technique.
2. Description of the Prior Art
Magnetic resonance imaging (MRI), also known as magnetic resonance tomography (MRT), is a known technique for obtaining images of the inside of the body of an object under investigation, such as a patient. For this purpose, a magnetic resonance apparatus has a space for receiving the examination subject. A basic field magnet system of the apparatus generates a static magnetic field in an imaging volume within the examination space that is as homogenous as possible. This basic magnetic field orders or aligns the nuclear spins of the nuclei in the subject, which were previously randomly oriented. A radio-frequency (RF) antenna system is also a part of the apparatus, and includes an RF transmission coil and at least one RF reception coil. In some instances, the RF transmission coil and the RF reception coil may be the same. RF energy is irradiated into the examination subject by the RF transmission coil, causing magnetic resonance signals to be generated in the subject, which are detected (received) by the RF reception coil or coils.
The received, analog magnetic resonance signals are converted into digital signals, and represent a so-called raw data set. The raw data set is stored electronically in a mathematical domain known as k-space. By means of a Fourier transformation, the data in k-space are transformed into image data, from which an image of a slice of the examination subject is reconstructed.
Particularly in the case of cardiac MRI, two problems are persistent, namely patient motion, including respiratory motion, during data acquisition, and a relatively long data acquisition time. Such problems also exist in other types of MRI such as, for example, functional MRI (fMRI).
Two prevalent techniques are currently employed for reducing motion artifacts in the image, namely the use of navigator echoes, and the use of a special k-space trajectory. The k-space trajectory defines the path or sequence that the raw data are entered into k-space. “Empty” k-space can be considered as a grid or raster or matrix with a number of locations that need to be filled (entered) with data. Many known MRI protocols fill k-space along a serpentine path proceeding row-by-row. For reducing motion artifacts, it is known to employ different types of k-space trajectories, such as radial and spiral trajectories. A known version employing a radial trajectory is called PROPELLER. The PROPELLER technique or method is described in “Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) MRI; Application to Motion Correction,” Pipe, ISMRM 1999, page 242; “Motion Correction with PROPELLER MRI: Application to Head Motion and Free-Breathing Cardiac Imaging,” Pipe, MRM Vol. 42 (1999), pages 963–969; “Multishot Diffusion-Weighted FSE With PROPELLER,” Pipe, ISMRM 2001, page 166 and “Turboprop-An Improved PROPELLER Sequence for Diffusion-Weighted MRI,” Pipe, ISMRM 2002, page 435.
The PROPELLER technique has proven to be a good solution for reducing motion artifacts, but it requires an additional π/2 imaging time, due to the necessity of oversampling the central region of k-space.
Moreover, two primary methods for speeding up image acquisition are known, these being partial k-space acquisition and parallel acquisition techniques (PAT). Parallel acquisition techniques employ multiple reception coils, usually arranged in a coil array, for simultaneously receiving the magnetic resonance signals from the subject. No single coil in the array receives sufficient signals (data) for reconstructing an entire image of the subject, and therefore it is necessary to combine the signals respectively received by the individual coils in order to produce data for the overall image. This in turn requires that information regarding the respective locations and/or sensitivities of the individual coils be known so that their respective data contributions to the overall image can be appropriately weighted.
Parallel acquisition techniques are described, for example, in “Recent Advances In Image Reconstruction, Coil Sensitivity Calibration, and Coil Array Design For SMASH and Generalized Parallel MRI,” Sodickson et al., MAGMA, Vol. 13, No. 3 (January 2002), pages 158–163; “Generalized SMASH Imaging,” Bydder et. al., Magnetic Resonance in Medicine, Vol. 47, No. 1 (January 2002), pages 160–170 and “SENSE: Sensitivity Encoding for Fast MRI,” Preussmann et al., Magnetic Resonance in Medicine, Vol. 42, No. 5 (November 1999) pages 952–962.
Thus far, however, parallel acquisition techniques have been successfully applied primarily in the case of rectilinear (row-by-row) k-space acquisition. The applicability of PAT to arbitrary k-space trajectories is under research, as described in “Advances in Sensitivity Encoding with Arbitrary k-space Trajectories,” Preussmann et al., Magnetic Resonance in Medicine, Vol. 46 (2001), pages 638–651.